The invention relates to the production of substantially pure polyhydric alcohols. More particularly, this invention relates to a single-step process for the catalytic production of substantially pure polyhydric alcohols from polysaccharides. A particular mode of the invention relates to the simultaneous hydrolysis and hydrogenation of polysaccharides, using an heterogeneous catalyst, to produce substantially pure hexitols.
The term "carbohydrate" as used herein includes monosaccharides, polysaccharides, and mixtures of monosaccharides and/or polysaccharides.
The term "polysaccharide" as used herein includes those saccharides containing more than one monosaccharide unit. This term thus includes disaccharides and oligosaccharides.
The supported nickel catalysts supported on carbon, diatomaceous earth or kieselguhr, as described in U.S. Pats. No. 3,538,019 and 3,670,035 (which is a division of U.S. Pat. No. 3,538,019) have high activity for the conversion of both monosaccharides and polysaccharides, including carbohydrate mixtures such as corn starch hydrolzate, with high selectivity to sorbitol when either corn starch hydrolyzate or dextrose is used as the starting material. A disadvantage of the catalysts described in U.S. Pats. No. 3,538,019 and 3,670,035 is that they cannot be regenerated; when reactivation is required, it is necessary to remove the active catalyst material from the support by chemical means and then to redeposit the catalyst metal on the support. Various other nickel catalysts for conversion of carbohydrates to polyhydric alcohols are cited in U.S. Pats. No. 3,538,019 and 3,670,035.
The conversion of carbohydrates to polyhydric alcohols using ruthenium on a solid carrier is known. U.S. Pat. No. 2,868,847 discloses the use of ruthenium on an inert catalyst support such as carbon, alumina, silica, or kieselguhr, as a catalyst for the catalytic hydrogenation of mono- and disaccharides, the latter being hydrolyzed and hydrogenated to hexitols. However, maltose, a disaccharide containing two glucose units, was more easily converted to maltitol, a C.sub.12 alcohol, according to the patent.
The hydrogenation of aqueous solutions of carbohydrates over zerovalent group VIII metals dispersed on alpha-alumina is disclosed in U.S. Pat. No. 4,380,680.
The hydrogenation of monosaccharides using a supported ruthenium, palladium, platinum, or nickel catalyst (activated carbon was used as the support in all experimental work) is discussed in an article by N. A. Vasyunina et al. "Catalytic Properties of Ruthenium in Monosaccharides Hydrogenation Reaction" in Izvestiya Akademii Nauk SSR Khimicheskaya Serya 4:848.854 (1969). Ruthenium was found to have higher activity than the other three catalysts.
A two stage process for hydrogenation of ligneous and other plant material such as wood sawdust is disclosed in Izv. Akad. Nauk SSR. Otd. Khim. 8:1522.1523 (1960). The process consists of a first stage hydrolytic hydrogenation of polysaccharides in an acid medium, followed by a second stage hydrogenation of the lignin in an alkaline medium using a ruthenium catalyst in both stages. In a specific embodiment, pine sawdust is treated using an aqueous phosphoric acid medium and a ruthenium-on-carbon catalyst. The first stage reaction product is filtered to separate the liquid medium from the crystals obtained from the first stage filtrate.
Belgian Patent No. 837,201 and U.S. Pats. No. 3,963,788, 3,963,789 and 4,072,628 disclose a two-stage high-pressure process for the conversion of carbohydrates to polyhydric alcohols in the presence of a ruthenium-containing catalyst prepared by depositing RuCl.sub.3 on a support which is either a zeolite catalyst having a Si/Al molar ratio greater than 3, or a clay. The catalyst can be regenerated by contacting it with an aqueous solution of a mineral acid.
A process for the continuous preparation of polyhydric alcohols over lumps of catalyst, made of ruthenium supported on animal coal, was disclosed in U.S. Pat. No. 4,520,211. However, a series of two consecutive reactors was necessary. Further, it was necessary to lower the pH of the aqueous carbohydrate solution to about 2.5 to 4.5.
However, the known procedures have serious disadvantages. First of all, the catalysts used therein cannot be regenerated. Also, they do not allow the preparation of polyhydric alcohols directly from polysaccharides and they must therefore use expensive pure monosaccharides instead of the less expensive polysaccharides.
Thus, although various catalytic processes for the conversion of carbohydrates to polyhydric alcohols are known in the art, none of them shows all desirable features, e.g. a single-step process combining hydrolysis and hydrogenation, substantial purity of the products, and no requirement for prior activation of the feedstocks. There is accordingly a need in the art for an improved process which would possess all these characteristics.